When a stranger threatened to carjack them, the nanny of super-trainers Chris and Heidi Powell of ABC's "Extreme Makeover Weight Loss Edition" fame said her initial instinct was to protect the couple's 1-year-old son, Cash, who was in the backseat.

"There was no question in my mind to save myself before Cash. It never crossed my mind to leave him or to stay in the car," Kaitlyn McGrath told "Nightline." "I don't feel like a hero. I just feel like I did the right thing."

Through tears, McGrath, a 20-year-old college student, detailed the harrowing ordeal of when she and the toddler were sitting in the Powells' Buick Enclave at Heidi Powell hairdresser's home in Mesa, Ariz., last Friday, while Heidi Powell was inside. She said she was on her phone and Cash was playing on Heidi's iPad when a man knocked on the car window.

"He told me to get out of the car or he was going to kill me and the baby," McGrath said. "When he got in the car that's when I knew we were in crisis, we were in trouble."

The suspect, who was later identified as 32-year-old Jasper Stewart, allegedly got into the car and McGrath said it took her a moment to realize what was going on. But within seconds, she said she knew the man was serious, but she remained calm.

"The only thing I ever said to him was 'excuse me,' wondering at first what was happening," she said. "After that, I just really went into the mode, the fighting mode. I knew I had to grab Cash, more than anything, and get us out."

McGrath's quick thinking likely saved them from being taken, police say. After the man got into the car, McGrath said she immediately leaned over, grabbed the baby and called 911 as she ran into the house to get Heidi.

"I was crying and I mean I almost couldn't catch my breath," McGrath said. "I was running on adrenaline. My stomach was in knots. And it was pure fear at first and the realization of what happened that made me very emotional at the moment."

Chris Powell told "Nightline" that he was in Milwaukee on a work trip when the carjacking happened, but was on the phone with Heidi when Kaitlyn came rushing into their home.

The suspect drove off with the couple's car, but technology quickly led to his capture when police said they used tracking software to find him and disable the vehicle. Powell said they called the police and the in-vehicle security company OnStar, who tracked the car, which ended up being about 2.5 miles away, and locked its ignition by remote control, all in the span of five minutes.

At that point, police said, the carjacker made a run for it, but his final mistake was stealing Heidi's iPad. Heidi and the police used the "Find My iPad" application, which located his whereabouts. Within half an hour, officers had the suspect in custody.

Jasper Stewart was arrested on robbery charges. Police said he has tattoos that are associated with the Nazi Low Riders, a skinhead group.

McGrath said she has received nothing but gratitude from the Powells for what she did. Chris Powell, who specializes in helping people overcome trauma, is now working overtime to heal those closest to home.

"[Chris] helped me realize that I shouldn't feel guilty and that I should feel proud," McGrath said. "For him to tell me that means the world for me, I love Chris and Heidi and they have been really greatest supporters in this. I'm so grateful for them."